This invention relates generally to a cable tensioner, and in particular to a cable tensioner for a window regulator device.
A window regulator is a device used in a motor vehicle fitted with a window to transmit a driving force to the window. The window regulator includes a drive component, such as a cable or a belt, connected to a driving mechanism, such as a crank or a motor, which moves the window. The window can be driven by slides that move along guide rails under the action of the cable. The cable can be split into a lower cable and an upper cable wound in opposite directions on a drum driven by the motor or the crank.
The tension of the cable in the window regulator must be well controlled throughout its service life. Slack in the cable loop causes inaccurate movement of the window relative to a position expected by the drive system.
The accuracy of the operation of the window regulator is important and particularly critical in window regulators used in frameless doors. In certain models of window regulator in frameless doors, the window lowers slightly when the door is opened to disengage from the roof seal. The movement of the window must therefore be accurate to not impede the opening of the door. However, this movement must not be too great either in order to comply with the requirements of the regulations, in particular regarding anti-pinch regulations. The drive cable of the window must therefore have sufficient tension to guarantee accurate movement of the window.
However, the components that make up the window regulator device undergo gradual ageing which leads to the apparent elongation of the cable due, for example, to wear of the driving drum and the pulleys or cable sheath shortening due to compression or pulley metal creep. The elongation of the cable due to the ageing of the different parts of the window regulator must be compensated for.
Play compensation mechanisms are known that absorb cable elongation to guarantee sufficient tension for the satisfactory operation of the window regulator device.
Most of the play compensation mechanisms are systems that operate according to a stepped mode, based on detents of greater or smaller size, such as the one disclosed in German patent application DE 197 06 866 A.
To guarantee greater uniformity in tension, it is preferable to use a continuous compensation system that avoids threshold effects. Such a mechanism is disclosed in patent application EP 0 244 303 and includes a cable tensioner with a screw and a nut that is acted upon by a push rod and a flexible component. In one direction, the screw can, under an axial thrust of the flexible component, carry out a helical movement in the nut each time the force exerted by the push rod disappears or is reduced. In the other direction, the screw cannot carry out any rotation or translation in the nut. This mechanism is an irreversible friction based system. Thus, when play is likely to occur between two parts linked to the tensioner screw, play is automatically compensated for as it occurs by the one-way movement of the screw.
However, this mechanism is not completely irreversible. For example, if grease or any other substance that reduces the coefficient of friction (for example coming from the cable or the drum of the window regulator) is introduced between the screw and the nut of the cable tensioner, the screw can then translate in the nut in the reverse direction. Thus, excessive tension in the cable, for example due to excess torque when the window is in the upper position or the lower position, can cause a reverse translation of the tensioner screw and an undesirable elongation of the cable.
There is therefore a need for a cable tensioner that compensates for play in a cable in a window regulator device and that ensures absolute irreversibility of the cable tensioner.